1. Field of the Invention
The present invention relates to a resin-coated carrier for an electrophotographic developer used in a two-component electrophotographic developer used in copiers, printers and the like, and an electrophotographic developer using the resin-coated carrier.
2. Description of the Related Art
A two-component electrophotographic developer used in electrophotography is composed of a toner and a carrier, and the carrier serves as a carrying substance to form a toner image on a photoreceptor in such a way that the carrier is stirred and mixed together with the toner in a developing device, to impart an intended charge to the toner, and conveys the thus charged toner to an electrostatic latent image on a photoreceptor to form the toner image on the photoreceptor. And the developer is repeatedly used while the developer is being replenished with an amount of fresh toner corresponding to the amount of the toner spent by the development.
Accordingly, the carrier is required to be able to stably impart charge to the toner in a long term independently of the environmental variation.
However, in a high-temperature and high-humidity environment, the charge amount is decreased, and hence problems such as toner scattering and fogging are caused, and additionally the charge is leaked, and hence, for example, there occurs a problem such that the electrostatic latent image is destroyed, and a problem such that the resistance of the developer is decreased to cause carrier beads carry over.
On the other hand, at a low temperature and a low humidity, the charge amount is increased, and hence the image density is decreased, and when the charge amount is extremely increased, at the time of the transfer of the toner to the photoreceptor, the carrier is also pulled to cause carrier beads carry over. The resistance of the developer is also increased, and hence the effective bias is decreased, to offer a cause for the occurrence of image density decrease and fogging.
Several proposals have hitherto been made for the purpose of making satisfactory such an environment dependence of the charge amount as described above.
Japanese Patent Laid-Open No. 06-324523 describes a carrier for an electrostatic image developer in which a coating resin is made of a polymer containing an alkyl methacrylate in a proportion of 50% by weight or more, and the carrier coating layer is made of resin fine particles having a water content of 0.10 to 1.0% by weight in a high-temperature and high-humidity, and which is used in combination with a negatively charged toner formed by a dry coating method. It is stated that according to the carrier for the electrostatic image developer, even when the carrier is used in a high-temperature and high-humidity environment, the degradation of the image quality is not caused, and an image stable and high in quality can be output.
Japanese Patent Laid-Open No. 2008-077002 also describes a carrier for electrostatic image development in which a coating resin layer is formed on the surface of magnetic substance particles, the coating resin layer includes a resin having a cycloalkyl group (preferably, a resin polymerized with 95 mol % or more of a monomer having a cycloalkyl group), and when the carrier is allowed to stand in an environment of 32° C./85% RH for 48 hours, the moisture content of the carrier is 0.05% by mass or less. It is stated that according to the carrier for an electrostatic image development, the leakage of the charge is effectively prevented, and a satisfactory charging property, in particular, a satisfactory charge rise property can be obtained.
However, when the carriers described in Japanese Patent Laid-Open Nos. 06-324523 and 2008-077002 are used, there occurs a problem such that the charge up at a low temperature and a low humidity comes to be large and the decrease of the image density and the fogging tend to occur.
Japanese Patent Laid-Open No. 2008-089925 discloses a carrier for electrophotographic development in which the surface of the particles of a carrier core material is coated with a resin, and the coating include conductive fine particles having a pH of 7 or more. It is stated that according to the carrier for electrophotographic development, it is possible to obtain a carrier coping with both of the environment dependence at a low temperature and a low humidity and the environment dependence at a high temperature and a high humidity.
Japanese Patent Laid-Open No. 2008-089925 states that the use of a conductive fine particle having a pH of 7 or less allows the aggregation of the carrier to be made to hardly occur; however, some degree of aggregation cannot be avoided, and a state of the conductive fine particle being exposed from the coating resin layer can be easily anticipated to occur.
Accordingly, even when the carrier of Japanese Patent Laid-Open No. 2008-089925 is used, as the coating resin layer is abraded, the degree of exposure of the conductive fine particle is increased so as to increase the charge leakage at a high temperature and a high humidity, and hence the carrier of Japanese Patent Laid-Open No. 2008-089925 cannot be said to be sufficient with respect to the environment dependence.
In Japanese Patent Laid-Open Nos. 06-324523, 2008-077002 and 2008-089925, no countermeasures are taken for the durability, it is difficult to maintain the initial environment dependence, and moreover, the abrasion or the exfoliation of the coating resin layer proceeds to a large extent and hence there is also a possibility that the original charge imparting property is lost.
When as has been done in Japanese Patent Laid-Open No. 2008-089925, additives such as a conductive particle are added, a problem such as the strength decrease of the coating resin or the detachment of the conductive particle occurs, thus the durability cannot be said to be sufficient, and as a result, it tends to be difficult to obtain stable images in a long term use of the carrier.